Summary: Enveloped viruses infect cells by fusing with cellular membranes. This fusion process is mediated by viral envelope glycoproteins (Env) when they undergo conformational changes while binding target cells. For many viruses, including HIV, Env binding to receptors on target cells triggers these fusion-inducing conformational changes. Our research aims to elucide the roles of CD4 and chemokine receptors in triggering conformational changes in Env required for membrane fusion. In particular, we are focusing on how Env binding to receptors affects the structure of the heptad repeat regions in the ectodomain of the transmembrane subunit of Env(gp41), regions known to be critical for HIV entry. Two heptad repeat regions (termed N- or C-heptad repeat according to proximity to the N-terminus of gp41) in the ectodomain of gp41 self-assemble into a thermostable six-helix bundle. This structure (trimer of hairpins)consists of a parallel coiled-coil trimer (N heptad) with three C-heptad helices packed in the grooves of the coiled coil in an anti-parallel fashion. Peptides corresponding to the N- or C-heptad repeat regions of gp41 are potent inhibitors of HIV infection. They are believed to prevent fusion-inducing conformational changes in gp41 in a dominant-negative fashion, by binding to gp41 and interfering with formation of the six-helix bundle required for fusion. We are using peptides to probe conformational changes in Env. Using a peptide corresponding to the C-terminal heptad repeat in the gp41 ectodomain (DP-178), we found that the peptide preferentially binds receptor-activated Env (see Furuta et al. 1998. Nat. Struct. Biol. 5: 276-279). Specifically, we found that CD4 is sufficient for triggering conformational changes in Env that allow DP-178 to bind gp41, but that binding to both CD4 and chemokine receptors may further enhance these changes. Because DP-178 likely binds to the N-heptad repeat region of gp41, these findings indicate both that a transient fusion-active conformation of gp41 can be a good target for inhibition and that the N-heptad repeat of gp41 changes conformation and/or becomes exposed after receptor activation. Current studies are aimed at refining our understanding of the receptor requirements for triggering specific conformational changes in gp41. We are also undertaking similar studies with another gp41 peptide corresponding to the N-heptad repeat region of the gp41 ectodomain (DP-107).